executor/mars: verify that the local and remote state of runners match

There are some attributes of a GitLab runner that cannot be modified using the gitlab-runner config file, such as tags or maximum timeout.

This can be problematic, as it means the tags associated with one DUT may become out of sync with the gitlab's runner, leading to jobs potentially being executed on the wrong machine...

This commit fixes this by introducing back the access tokens which, if present, allows accessing a runner's detail and comparing the tags with the local values. If any difference is found, the runner will be removed and re-registered.

Closes: #105 (closed)